Ever since the development of the first commercial jet transport, aircraft and engine manufacturers have been investigatiing ways of reducing noise caused by such aircraft. Since turbofan engines are big contributors of noise, various sound absorbing techniques have been applied to them. In one instance acoustic linings in the bypass duct and compressor casing of a jet engine are used to attenuate noise as disclosed in U.S. Pat. No. 3,640,357 for Acoustic Linings issuing Feb. 8, 1972 and assigned to Rolls Royce Limited. This lining comprises an impervious sheet, a channel layer, a first porous sheet, a honeycomb layer and a second porous sheet. Another by Rolls Royce Limited, U.S. Pat. No. 3,670,843 issuing June 20, 1972 for Sandwich Structures relates to a sound absorptive liner for use in gas turbine engines consisting of a sandwich structure which has two layers of cellular material separated by a porous sheet and an impervious sheet attached to the outer face of one of the layers of cellular material. A Sound Energy Absorbing Apparatus, U.S. Pat. No. 3,439,774 issuing April 22, 1969 and assigned to The Boeing Company relates to a sound absorbing panel comprising two spaced sheets of microporous material, the outer sheet being of high permeability for high frequency noise absorption, and the inner sheet being of a lower permeability for low frequency sound absorption, with cellular structure placed between the two sheets and between the inner sheet and a supporting surface. Still another U.S. Pat. No. 3,481,427 for Acoustical Panel Structure issuing Dec. 2, 1969 and assigned to McDonnell Douglas Corporation, the present assignee, uses the concept of transmission loss for sound energy absorption. Sound waves propagating over a porous surface pass into still air cavities where they are entrapped within the absorptive material below the open face.
All of the foregoing techniques absorb or attenuate sound through the use of multiple layers of material spaced by cellular cavities into which at least some of the sound energy is trapped. While these techniques are effective in attenuating low amplitude sound, a different physical principle underlies the absorption of high amplitude sound and shock waves to which the present invention is directed.